1. Field of the Invention
This invention relates generally to bias generator circuits, and more particularly to a bias generator circuit architecture and method that protects output skew voltage capabilities for an associated output circuit.
2. Description of the Prior Art
Known bias generator circuits are problematic in that they generate bias voltages to their associated output circuits that are inordinately sensitive to process, temperature and supply voltage variations, thereby limiting overall circuit performance capabilities. When the process is strong, temperature is low and supply voltage is at the maximum level, the output low-to-high and high-to-low propagation delays associated with an output circuit that is biased via the bias generator circuit are at their fastest. When the process is weak, temperature is high and supply voltage is at the minimum level, the output low-to-high and high-to-low propagation delays associated with an output circuit that is biased via the bias generator circuit are at their slowest.
FIG. 1A is a schematic diagram illustrating a known output voltage bias generator 10 that supplies different voltage levels (biasp 12 and biasn 14) to an output circuit 100 illustrated in FIG. 1B, wherein the different voltage levels are sensitive to variations in the process, ambient and operating temperatures, supply voltage and resistor tolerance. The bias generator 10 in combination with output circuit 100 promotes tight low-to-high and high-to-low output skews over the variation of operating conditions.
When the process is strong, temperature is low and supply voltage is at the maximum level (herein referred to as the first operating condition), the biasp 12 level is at a higher voltage level and the biasn 14 level is at a lower voltage level than they would otherwise be when the process is weak, temperature is high and supply voltage is at its minimum level (herein after referred to as the second condition). During a first condition situation therefore, the biasp 12 and biasn 14 voltage levels are closer to the thresholds of the skew adjusting devices MNSKEW1 (102) and MNSKEW2 (104), and will weaken the skew adjusting devices 102, 104. This will slow down the low-to-high and high-to-low propagation delays, pushing the skew closer to the second operating condition.
During a second condition situation, the biasp 12 level is at a lower voltage level than it would otherwise be during a first condition situation; and the biasn 14 level is at a higher voltage level than it would be during a first condition situation. During a second condition situation therefore, the biasp 12 and biasn 14 voltage levels xe2x80x9cturn onxe2x80x9d the skew adjusting devices 102, 104 harder than during a first condition situation, and will not weaken the skew adjusting devices 102, 104 as much as that caused during a first condition situation. The foregoing described varying voltage levels for biasp 12 and biasn 14 will cause the low-to-high and high-to-low skews to become tighter over process, temperature and supply voltage extremes.
The output voltage bias generator 10 is problematic however, in that it can get into a condition (variation in process, temperature, supply voltage and resistor tolerance) where the biasp 12 and biasn 14 voltage levels supplied to the skew adjusting devices 102, 104 in the output circuit 100 are at a xe2x80x9cchoking offxe2x80x9d voltage level (i.e. weakening the skew devices 102, 104 by supplying biasp 12 and biasn 14 voltage levels to their respective gates that are too close the operating voltage thresholds). Although a designer can adjust certain parameters associated with the output voltage bias generator 10 to compensate for specific conditions, such adjustments will degrade the skew adjusting capabilities of the output voltage bias generator 10, and therefore reduce the usefulness of the output voltage bias generator 10.
FIG. 2 is a waveform plot diagram 200 illustrating input and output waveforms 202, 204 associated with the output voltage bias generator 10 and the output circuit 100 with a supply voltage of 3.3 volts at a nominal operating temperature of 25xc2x0 C. Two operating conditions can be seen to need assistance. The first such operating condition is associated with a strong process coupled with a nominal resistance variation. The second such operating condition is associated with a strong process and a weak resistance variation. It can be appreciated that similar assistance is also necessary for other variations associated with process, temperature, supply voltage and resistor tolerance.
In view of the foregoing, a need exists for a bias generator and associated output circuit architecture that protects output skew voltage capabilities for the associated output circuit to a greater extent than that achievable using presently known circuit architectures.
The present invention is directed to a method as well as a bias generator and associated output circuit architecture that protects output skew voltage capabilities for the associated output circuit to a greater extent than that achievable using presently known circuit architectures. A differential-pair circuit detects bias voltage levels provided by the bias generator and provides a signal to skew adjusting assist devices within the associated output circuit when the bias voltage levels get close to a xe2x80x9cchoking offxe2x80x9d voltage level. The signal turns on the skew adjusting assist devices to assist the skew adjusting devices.
According to one aspect of the invention, an improved output skew voltage bias generator is provided to enhance an existing circuit in achieving its skew adjusting potential without concern for the existing circuit defaulting to a condition with very slow propagation delay results.
According to another aspect of the invention, an improved output skew voltage bias generator is provided to enhance existing circuit manufacturing yields.
According to yet another aspect of the invention, an improved output skew voltage bias generator is provided to enhance an existing circuit in maximizing its skew adjusting potential such that the existing circuit will have smaller skew variations.
One embodiment of the present invention comprises a voltage bias generator comprising:
an output circuit having a plurality of voltage skew adjusting devices and a plurality of voltage skew assist devices, the output circuit responsive to an input signal to generate an output signal having a desired skew;
a bias voltage circuit configured to generate desired bias voltage signals for the plurality of voltage skew adjusting devices; and
a voltage sensing circuit responsive to at least one of the desired bias voltage signals to generate desired bias voltage signals for the plurality of voltage skew assist devices such that the output signal will have the desired skew.
Another embodiment of the present invention comprises a voltage bias generator comprising:
voltage skew adjusting means for adjusting an output skew voltage associated with an output signal in response to an input signal;
voltage skew assisting means for assisting the voltage skew adjusting means such that the voltage skew adjusting means will not be choked off during at least one marginal operating condition selected from the group consisting of temperature, process, supply voltage and resistor tolerance;
biasing means for biasing the voltage skew adjusting means; and
voltage detecting means for detecting bias voltage output signals associated with the biasing means and for generating desired bias voltage signals for the voltage skew assisting means such that the output signal will have a desired skew.
Yet another embodiment of the present invention comprises a voltage bias generator operational to sense an input voltage signal and generate an output signal having a desired voltage skew, the voltage bias generator comprising skew adjusting devices responsive to first internally generated bias signals and further comprising skew assist devices responsive to second internally generated bias signals to assist the skew adjusting devices control the desired voltage skew such that the skew adjusting devices are not choked off during at least one marginal operating condition selected from the group consisting of temperature, process, supply voltage and resistor tolerance.
Still another embodiment of the present invention comprises a method of controlling output skew voltage, the method comprising the steps of:
providing a voltage bias generator comprising skew voltage adjusting devices and skew assist devices;
providing an input signal to the skew voltage adjusting devices;
processing the input signal via the skew voltage adjusting devices to generate an output voltage signal having a desired skew; and
adjusting process control characteristics associated with the skew voltage adjusting devices via the skew assist devices such that the skew adjusting devices are not choked off during at least one marginal operating condition selected from the group consisting of temperature, process, supply voltage and resistor tolerance.